Engineered air rooftop units

Engineered air pre-purge error
Combustion motor on engineered air units are variable speed, which are controlled by the hertz to the motor. Voltage will always remain constant and to slow down the motor the DJM2 controller will run between 15-59 Hz. If the reading it over 59 Hz then the speed sensor is grounded and the wire and/or sensor need to be replaced, 3450 RPM equals 59 Hz.

  • If combustion motor is running first check tachometer signal, Y to G should run between 4-6 VAC. If AC is not present check that the sensor is 1/16 inch from magnet at the end of combustion motor shaft. Also insure the sensor is not over the centre of the shaft, But rather it is off to one side of the magnet.
  • If voltage is within 4-6 VAC, The DJM2 is sensitive to the shape of the waveform produced by the speed sensor. You will need a fluke 80 series similar multi meter that can measure percent duty cycle. Connect meter to Y and G press Hz for Hertz then second time for duty cycle. Speed sensor will output a signal close to 50% duty cycle and must be accurate within 2.5%. If the number is outside those parameters the pre-purge problem light will flash
  • Many time the only thing wrong with this system is the magnet at the end on the motor shaft. By flipping over the magnet sometime will fix the problem



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